Tunable add-drop and cross connect devices include materials with negative refractive index dependence on temperature and temperature independent coincidence between modes of an athermal ring resonator and a set of specified frequencies, e.g. for DWDM telecommunications channels. The free spectral range may be adjusted to equal a rational fraction of the specified frequency interval. The operating frequency may be selected without substantially tuning the resonator modes by a thermo-optically tuned feedback element having a waveguide pair with differential thermal response. The operating frequency may be induced to hop digitally between the specified frequencies.